Facial recognition systems in use at the British entry gate of the Eurostar railway in Paris in February.

Airports have been collecting biometric data on most non-U.S. citizens for years by taking photos at Passport Control when they enter the country and checking them against government databases, Jennifer Gabris, a Customs and Border Protection spokeswoman, told HuffPost.

In most cases, our federal government simply depends on good faith compliance by nonimmigrants (usually tourists, business travelers, and guest workers) to ensure that individuals honor the terms of their admission and turn in their departure form. Since an estimated 40-50 percent of unlawfully present foreign citizens (or 4.5-6 million) entered the United States legally and failed to leave the country when required, it is clear that operating on good faith is not the best way to ensure the integrity of our immigration system.

So the government is looking for a way ― probably using facial recognition, but potentially using eye scans or other measures too ― to verify which travelers have left the country by collecting biometric data right before passengers board a flight. This isn’t a new idea: Many countries already use face scans extensively in their airports and train stations, and the U.S. has been working on its own way to track exiting travelers for more than 20 years. However, CBP is now under extra pressure to choose a method and get a system into airports, said Theresa Cardinal Brown, an immigration expert at the Bipartisan Policy Center.

So yes, more face scans are coming soon, even for U.S. citizens.

In 2015, CBP started piloting biometric exit programs in America’s 10 busiest airports by using fingerprint scans. But the agency chose facial recognition as one of the easiest ways to do so, Gabris said, and will move forward with that.

Last year, CBP started piloting face-scanning technology on some travelers exiting Hartsfield–Jackson Atlanta International Airport. Pilot programs will be added to seven more airports in June, though the exact locations can’t be disclosed yet, Gabris said.

These pilot programs also scan the faces of U.S. citizens, but Gabris said that once their citizenship is confirmed, their data is deleted, while non-citizens’ data is kept on file. She said the same will be true of whatever biometric exit system becomes permanent.

ERIC PIERMONT via Getty Images

A traveler tries out a biometric scanning system at the Paris Air Show.

This push didn’t start with Donald Trump, but he wants it to happen soon.

Bill Clinton’s 1996 immigration act called for an automated entry-exit system to track travelers, and later bills named biometrics as a way to do so, Brown said. Progress continued off-and-on during subsequent administrations and has been moving more quickly since 2013, when CBP was officially put in charge of the biometric exit project.

There’s some concern, however, that Trump’s advisors will disregard previous work on the project, failing to examine how their collection of personal data jives with U.S. privacy law, said John Cohen, former counterterrorism coordinator at the Department of Homeland Security.

Groups like the American Civil Liberties Union have called biometric data collection a threat to privacy. What’s more, U.S. facial recognition systems have higher rates of error in recognizing minorities, because much of the technology was developed using Caucasian faces in testing. Using it in airports could increase racial profiling of passengers, said Rich Stolz, executive director of OneAmerica, a civil rights group that advocates for immigrants.

In Cohen’s opinion, the Trump administration would be wise to address concerns like these.

“The current administration is still in a learning curve about these issues,” he said. “I believe the attitude in the White House is they’re resistant to talk with individuals outside their circle who have been involved with security over the last few years... Hopefully they’ll reach out to those experts.”

It’ll be a while before face scanners are in every airport, if they make it to every airport at all.

Making facial recognition the official technology for biometric exit will be expensive, said Cohen. In order for the system to work to its full capacity, CBP will have to install face-scanning cameras in every airport, land border and seaport in the country.

Finding a way to take exit photos without causing travel delays will cost even more.

“[CBP] wants passive technology that can grab data as people walk by,” Brown said. “The most efficient way is literally on the jetway, as people are boarding a plane... But we’ve never built airports to do that.”

Cohen said he thinks U.S. airports can make better use of the information they already collect ― like entry photos and forms ― to achieve the same goals as a biometric exit program that uses facial recognition, he said. To him, a complete overhaul isn’t necessary, never mind feasible.

Indeed, said Gabris, “there are many more infrastructure and process challenges that exist for biometric exit.”